Mechanical pulping is the most efficient method of producing pulp from wood with a yield from raw material of greater than 95%. The use of pulpstones is the oldest and most energy efficient method for the production of mechanical pulp from wood. Pulp produced from wood grinding usually contains more so called fines than pulps from other modes of pulping, resulting in paper with superior properties (e.g., higher opacity and better printability).
In operation, the abrasive grits in the pulpstone pass over fibers in the wood. The compression and release action that results from the grits passing over the fibers, combined with the heat of the process, softens the lignin bond around the fibers. As is known, lignin is the naturally occurring binding agent in wood that that holds cellulose fibers together. As the lignin softens, sheer forces exerted by the grits operate to separate and peel fibers from the wood matrix as the pulpstone rotates. In general, the wood being pulped or defibrated (typically in the form of logs) has a horizontal orientation that is parallel to the axis of rotation of the pulpstone as it is fed onto the pulpstone, and the fibers come away in the transverse direction of rotation. The sheer and consequent peeling are not precisely perpendicular to the orientation of the wood. Rather, the angle of fiber peeling is a function of the pattern on the pulpstone, one of the ways fiber characteristics can be controlled.
One drawback of pulp produced by such conventional processes is the lack of paper strength due to relatively short fibers. The fibers are short, because they tend to break during the mechanical defibration process. To compensate for this drawback, the pulp furnish is typically mixed with chemically produced pulp, which contains longer fibers and provides strength to the paper. However, such chemically produced pulp increases pulp costs and is manufactured by processes that are associated with environmental issues.
There is a need, therefore, for pulp production techniques that produce long fiber pulp.